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Press2ToContinue writes "According to a NASA news release, 'Twin lunar-orbiting NASA spacecraft that have allowed scientists to learn more about the internal structure and composition of the moon are being prepared for their controlled descent and impact on a mountain near the moon's north pole at about 2:28 p.m. PST (5:28 p.m. EST) Monday, Dec. 17. Ebb and Flow, the Gravity Recovery and Interior Laboratory (GRAIL) mission probes, are being sent purposely into the lunar surface because their low orbit and low fuel levels preclude further scientific operations. The duo's successful prime and extended science missions generated the highest resolution gravity field map of any celestial body. The map will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved. Both spacecraft will hit the surface at 3,760 mph (1.7 kilometers per second). No imagery of the impact is expected because the region will be in shadow at the time.' That's too bad; observing the impacts could provide valuable feedback. For example, a spectrographic analysis of the impact dust cloud could reveal additional density and compositional element information for the lunar polar surfaces."
Emily Lakdawalla at the Planetary Society has more information about the violent end to GRAIL's mission. If the probes were going to hit the surface of the Moon vertically, they would probably leave a crater about 3 or 4 meters in diameter. However, they are actually coming in at a very slight angle: 1.5 degrees from the horizontal, though the mountain itself has a 20-degree slope. Despite the darkness at the impact site, NASA will attempt to monitor the crashes using the Lunar Reconnaissance Orbiter.